The Great Dark Spot

The Cassini spacecraft has photographed an extraordinary
dark cloud on Jupiter twice as big as Earth itself.

March 12, 2003: For more than
a century astronomers thought that the Great Red Spot was the
biggest thing on Jupiter. Not anymore. Images from NASA's Cassini
spacecraft have revealed something at least as large.

The Great Dark Spot.

"I was totally blown away when I saw it--a dark cloud
twice as big as Earth swirling around Jupiter's north pole,"
says Bob West, a planetary scientist at the Jet Propulsion Laboratory.

West has been chasing this cloud for some time. He first saw
it--"just a glimpse," says West--in an ultraviolet
(UV) picture of Jupiter taken by the Hubble Space Telescope in
1997. But it only appeared in one image out of many spanning
a period of years. "I didn't know what to make of it,"
he recalls.

Now he knows. "The Cassini spacecraft was en route
to Saturn in 2000 when it passed by Jupiter and had a good view
of the planet's north pole," says West. "At first there
was nothing unusual--just ordinary polar clouds. Then the Dark
Spot emerged." For weeks Cassini's UV-sensitive cameras
watched as the cloud grew into an oval the size of the Great
Red Spot itself. It swirled, darkened and changed shape until,
as Cassini was departing, it began to fade again. (See
the entire movie.)

"The Dark Spot is ephemeral," says West. That's
probably why Hubble saw it only once. And if Cassini had arrived
a month or two later, it might not have seen the Dark Spot at
all. Instead, Cassini's cameras monitored the cloud for 11 straight
weeks, and those data have allowed West to draw some conclusions:

"The
Great Dark Spot and the Great Red Spot are entirely different,"
he says. The Great Red Spot is deep. "It's a high-pressure
storm system rooted in Jupiter's troposphere far below the cloudtops.
The Great Dark Spot is apparently shallow and confined to Jupiter's
high stratosphere."

Left: The Great Red Spot is a long-lasting storm rooted
deep in Jupiter's atmosphere. [more]

West believes the Dark Spot is
a curious side-effect of auroras on Jupiter.

"Jupiter has Northern Lights
just as Earth does, although on Jupiter they are hundreds to
thousands of times more powerful," says West. Auroras happen
when electrons and ions rain down on the polar atmosphere and
cause the air to glow where they hit. Here on Earth, auroras
are usually sparked by solar wind gusts. The solar wind can also
trigger auroras on Jupiter, but it's not necessary: On Jupiter,
the planet itself energizes Northern Lights. "Jupiter's
magnetic field is a huge reservoir of charged particles,"
explains West. "These particles are accelerated poleward
by the 11-hour rotation of Jupiter and its magnetic field. Thus,
auroras on Jupiter are almost always active."

"High-energy electrons that
hit Jupiter's atmosphere not only cause auroras, but also break
apart methane (CH4), which is more abundant on Jupiter
than it is on Earth," says West. "Fragments of methane
molecules combine with ambient hydrogen to form acetylene C2H2.
That's the basic building block. Acetylene combines with other
carbon- and hydrogen-containing molecules to build even more
complex molecules, which eventually condense into dark droplets."

If West is right, the Great Dark
Spot is a haze of hydrocarbon-rich droplets floating in the uppermost
layers of Jupiter's stratosphere. Such a haze would be prominent
in UV images because hydrocarbon droplets are strong absorbers
of UV radiation. Indeed, the Great Dark Spot is invisible to
the human eye. "It can only be seen in UV light."

His idea fits the facts, but
West remains puzzled: "There was no strong auroral display
when the Great Dark Spot intensified in late 2000. What caused
it to appear when Cassini was flying by? We don't know. This
shows us that Jupiter's stratosphere is a more interesting place
than we once thought."

Furthermore, it can teach us
something about our own planet.

West
explains: "This dark spot is trapped by a polar vortex--a
jet stream that encircles Jupiter's north pole." Fast-moving
winds in the vortex act like an atmospheric wall, keeping the
Dark Spot corralled at high latitudes. Similar
vortices encircle Earth's polar regions. Our planet's Arctic
vortex is disrupted somewhat by northern land masses, but the
Antarctic vortex is better organized. It plays a key role in
confining the ozone hole--much as Jupiter's polar vortex confines
the Great Dark Spot.

"Monitoring the Dark Spot
could help us understand how planetary vortices work." For
such studies, two planets are clearly better than one.

Meanwhile, West would be delighted
just to see the Dark Spot again. "It's elusive," he
says. But he's ready to be blown away ... any time.

Web Links

Puzzling
X-rays from Jupiter
-- (Science@NASA) The Great Dark Spot isn't the only odd thing
at Jupiter's north pole. Astronomers using the Chandra X-ray
Observatory have spotted a mysterious pulsing x-ray beacon there,
too.

How old is the Great Red Spot? It's often said to be at least 300 years
old, and it may be. But documentation of it is questionable prior
to 1879. In Time-Variable Phenomena in the Jovian System
(NASA SP 494, 1989) Beebe, Orton and West (p.261) state, "Reports
of red spots extend back to the 17th century. Some historical
records indicate that the Great Red Spot was first observed in
1879. The question of whether the GRS has been continually present
is more difficult to answer...."